docs - add ng add as alternative to npm/yarn

[elAndyG]

Letting users know that the ng add command will simplify the install process if they are not on the latest versions of Angular. I found that by doing the stock npm install, the @angular/cdk and @angular/animations versions did not match what was already installed causing conflicts in the dependencies.

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[elAndyG]

@googlebot I signed it!

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CLAs look good, thanks!

[crisbeto]

LGTM

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[jelbourn]

For 7.0, I want to actually change the getting started guide completely to use ng add as the primary workflow

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